Open-source browsers Mozilla and Firefox have won over a significant number of defectors from Microsoft's Internet Explorer in the past nine months, Web site metrics suggest.

The gains for Firefox, which was released in a version 1.0 preview on Tuesday, and for Mozilla are most noticeable at Web sites popular by geek-chic early adopters. For example, W3Schools.com, a Web development tutorial site found that Mozilla-based browsers were used by 18 percent of its visitors in September, up from 8 percent in January. Internet Explorer use dropped to 75 percent from 84 percent in the same period.

Among CNET News.com readers, site visitors with the Firefox and Mozilla browsers jumped to 18 percent for the first two weeks of September, up from 8 percent in January.

The Mozilla Foundation, which develops both the Mozilla and Firefox browsers and their underlying Gecko browsing engine, has noted that downloads of the Firefox browser have doubled from 3 million for version 0.8 to 6 million for version 0.9. The group said that almost 160,000 people have downloaded the latest version of Firefox, the 1.0 release candidate.

"Obviously, there was a big spike in interest back in July when there were concerns with Internet Explorer security," Mozilla spokesman Bart Decrem said. "You worry that such things are just a blip, but that momentum has seemed to continue for us."

Mainstream users have not shown the same gung-ho enthusiasm for the non-Microsoft browser but have increasingly adopted Firefox, according to Web analytics firm WebSideStory. The percentage of visitors to e-commerce and corporate sites that used Firefox or another Mozilla browser grew to 5.2 percent in September, from 3.5 percent in June 2004. Meanwhile, Microsoft's share of the users shrank from 95.5 percent in June to 93.7 percent in September, according to the company.

Launched by Netscape Communications in 1998, the Mozilla Foundation has not done well in its head-to-head battle with Microsoft until recently. In 2000, Netscape introduced a browser based on the group's open-source development efforts, but that release tanked. Code bloat cost the Mozilla Foundation support for its browser until it focused on producing a smaller and faster application, dubbed Phoenix, then Firebird and finally Firefox to avoid naming issues.

Mozilla's software has benefited from several major security issues that have plagued Microsoft's Internet Explorer. In June, security researchers found that aggressive advertisers were using a flaw in Internet Explorer to surreptitiously install adware on victim's PCs. This week, another graphics flaw could also cause trouble for Internet Explorer users.

Another factor in the open-source group's favor is that its software has some additional features, such as pop-up blocking, that Microsoft only recently added to its browser.

Firefox isn't yet a focus for Martin Taylor, the general manager of Microsoft's platform strategy, who is in charge of Microsoft's response to Linux and open-source software. But he suspects it will come under discussion in coming months.

"Firefox is creeping up, getting more attention," Taylor said in an interview on Wednesday.